Lithography is a process of transferring desired patterns onto a substrate (typically a target area of the substrate) such that patterns are created in different device regions or current regions. Specifically, using exposure in a lithography process, a pattern can be created onto a photoresist layer (made of a photo-sensitive polymer material) disposed on surface of the substrate to achieve pattern transfer.
With rapid development of semiconductor manufacturing technologies, critical dimensions (CDs) of patterns exposed by the lithography process have been reduced, which requires high resolution of lithography. The lithography resolution, or the minimum critical dimension of lithography, is given based on the Rayleigh's criterion, as shown in equation (1):CD=κ1×λ/NA  (1)where λ is exposure wavelength of the lithography process; NA is numerical aperture of the projection system of lithographic equipment; κ1 is a lithography process-related factor; and CD is the critical dimension of the printed pattern. According to the above equation (1), CDs can be reduced by three methods, i.e., reducing the exposure wavelength, increasing the numerical aperture, or decreasing the κ1 factor.
EUVL has been considered the most promising lithographic technology. EUVL radiation is an electromagnetic radiation having a wavelength ranging from 5 nm to 20 nm and is currently generated by either laser-produced plasma (LPP) or discharge-produced plasma (DPP).
EUVL light source system for generating EUV light usually includes a source-excitation module for generating EUV-light-producing plasma from a vaporized source material, and a collector module for collecting and collimating the appropriate EUV light generated from the EUV light source-excitation module into an optical non-tele-centric system. In a laser-produced plasma system, the source-excitation module usually applies high-energy laser beams to the source material which then produces plasma in the excitation source. In a discharge produced plasma system, high voltage produces plasma which generates EUV light from the excitation source. The collector module has a number of optical elements used to direct, select and collimate the EUV light at a desired wavelength into an output EUV beam.
However, when a conventional EUV light source system excites the solid source material into vapor which then forms EUV-producing plasma, the source material vapor droplets may condense on the EUV light-collecting optical elements. As a result, these condensed droplets can contaminate EUV light reflecting optics in the light source system. In addition, the downstream EUV-collecting optics can be contaminated by the flying-over droplets. Once contaminated, light-collecting efficiency goes down quickly. The disclosed methods and systems are directed to solve one or more problems set forth above and other problems.